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Cont,d Muscle contraction : sliding of thin filaments over and b/n thick filaments by the action of numerous cross bridges that extend out from the myosine toward the actin. – : muscle fiber stimulation by motor neuron → ca +2 moves from its store site in the sarcoplasmic reticulum to the sarcoplasm→ attaches to troponine → conformational change that moves the troponin complex and its attached tropomyosine out of the way so that cross bridges can attach to actin→ sliding of filaments and thus muscle tension and shortening. ▪ Energy source : fatty acids, glycogen and blood glucose, aminoacids : skeletal muscles generate ATP through anaerobic and aerobic respirations and phosphate groups donated by creatine phosphate. ATP : serves as the immediate source of energy for 1. movement of cross bridges for muscle contraction 2. pumping of ca +2 in to the sarcoplasmic reticulum for muscle relaxation.

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TABLE Observations on Examination That Disclose Muscle Weakness Functional ImpairmentMuscle Weakness Inability to forcibly close eyesUpper facial muscles Impaired puckerLower facial muscles Inability to raise head from prone positionNeck extensor muscles Inability to raise head from supine positionNeck flexor muscles Inability to raise arms above headProximal arm muscles (may be only scapular stabilizing muscles) Inability to walk without hyperextending knee (backkneeing or genu recurvatum) Knee extensor muscles Inability to walk with heels touching the floor (toe walking) Shortening of the Achilles tendon Inability to lift foot while walking (steppage gait or footdrop) Anterior compartment of leg Inability to walk without a waddling gaitHip muscles Inability to get up from the floor without climbing up the extremities (Gowers' sign) Hip muscles Inability to get up from a chair without using armsHip muscles

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Muscular dystrophies Group of inherited disorders characterized by progressive degeneration of groups of muscles, sometimes with involvement of the heart muscle or conducting tissue, and other parts of the nervous system. Classified based on the age at onset, distribution of affected muscles and pattern of inheritance. Duchenne muscular dystrophy Inheritance- X- linked recessive disorder Defective gene- Dystrophin Onset- usually b/n 3-5yrs age C/F – progressive weakness of the girdle muscles - difficulty running, jumping, hopping, unable to get up from the floor (Gower’s maneuver) - toe walking is associated with lordotic posture - contractures( hip, knee, elbow, wrist) with chest deformities →severe pulmonary infections → death at age 16-18yrs

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Endocrine and metabolic myopathies Many cause weakness Fatigue is more common than weakness Cause : not well defined : ? Disease of muscle or ? another part of the motor unit - CK : usually N, except in hypothyriodism - muscle histology : atrophy rather than destruction of the muscle fibers Rx- nearly all respond to treatment